Semiconductor input protection circuit

Abstract

A lateral PNP transistor PB and a lateral NPN transistor NB are serially connected between an input terminal and a reference potential (ground potential). In the transistor PB, a diode D 1 is formed. In the transistor NB, a diode D 3 is formed. When an ESD of +2000 V is input, the transistor NB turns on, whereas when an ESD of −2000 V is input, the transistor PB turns on. The level of a positive signal capable of being input is limited by the inverse breakdown voltage (e.g., 18 to 50 V) of the diode D 3 , whereas the level of a negative signal capable of being input is limited by the inverse breakdown voltage (e.g., 13 to 15 V) of the diode D 1 .

1. An input protection circuit comprising:
 a semiconductor substrate of a first conductivity type provided with a circuit to be protected; 
 an input terminal formed above the semiconductor substrate for supplying an input signal to the circuit to be protected; 
 a first well region of the first conductivity type, formed in one principal surface area of said semiconductor substrate; 
 a second well region of a second conductivity type opposite to the first conductivity type, formed in the principal surface area of said semiconductor substrate, and forming a PN junction with said semiconductor substrate; 
 third and fourth well regions of the second conductivity type formed in said first well region, and forming a first lateral bipolar transistor with a portion of said first well region serving as a base, the bottoms of said third and fourth well regions forming PN junctions with said first well region or with said semiconductor substrate and the third and fourth well regions not being part of a MOS transistor; and 
 first and second impurity doped regions of the first conductivity type, formed in said second well region and forming a second lateral bipolar transistor with a portion of said second well region serving as a base, 
 wherein said input terminal is connected to said third well region, said fourth well region and the base of said first lateral bipolar transistor are connected to said first impurity doped region, said first lateral bipolar transistor operates without a fixed base bias, and said second impurity doped region and the base of the second lateral bipolar transistor are connected to a reference potential node. 
 
   
   
     2. The input protection circuit according to  claim 1 , further comprising a current limiting resistor formed on an insulating layer formed in the principal surface area of said semiconductor substrate, wherein said input terminal is connected via said current limiting resistor to said third well region. 
   
   
     3. The input protection circuit according to  claim 1 , wherein the second lateral bipolar transistor is turned on to protect the circuit to be protected when a high negative bias voltage is applied to the input terminal. 
   
   
     4. The input protection circuit according to  claim 1 , wherein the first lateral bipolar transistor is turned on to protect the circuit to be protected when a high positive bias voltage is applied to the input terminal.